Rydberg atoms are atoms with an electron in a highly excited quantum state that is loosely bound to the atomic nucleus, rendering them exquisitely sensitive to electromagnetic fields. Optical techniques based on quantum-optical phenomena, such as electromagnetically induced transparency (EIT), are used to excite and measure these exotic quantum states to exploit their large electric polarizabilities, long coherence times, and closely spaced energy levels in measurements of electric fields to high levels of sensitivity for a wide range of microwave frequencies. Rydberg sensing with atomic vapors has shown great potential to realize new capabilities in metrology, sensing, and communications due to their compact size, high sensitivity, and versatility in microwave signal reception via microwave-to-optical conversion.

Rydberg sensor configurations generally involve multiple electromagnetic fields that connect multiple atomic states, using at least two or more optical fields for EIT and one or more microwave fields to be detected. The specific configuration choice of atomic states and optical wavelengths directly determines both sensing performance as well as the feasibility of realizing a practical sensor device depending on the complexity of laser requirements, thus requiring consideration and study of alternate configurations and evaluation of their performance.

In this study, researchers Brekke and Umland from St. Norbert College explore an unconventional ‘inverted’ Rydberg EIT configuration in an atomic vapor, wherein the optical probe field has a shorter wavelength than the optical coupler field in a two-photon Rydberg configuration. In such an inverted scheme, the EIT feature will generally be smaller but can offer sensitivity to other phenomena, such as the decays from the intermediate state and optical linewidth, and is compensated for in their measurements by using both optical absorption and fluorescence detection. The work demonstrates a microwave field measurement using the inverted scheme and explores the use of a detuned microwave field, which substantially shifts the energy of Rydberg states due to the AC-Stark effect. The measured AC-Stark shifts are in good agreement with theoretical predictions, validating Rydberg sensing approaches using off-resonant microwave-induced shifts. The microwave detuning combined with a detuning of the coupler laser from the Rydberg resonance can be conveniently varied to prevent unwanted excitation to the initial Rydberg state, potentially useful for coherent sensing and microwave-to-optical frequency conversion.

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